In the prior arts, there have been proposed various ignition systems for an internal combustion engine in which an ignition preventing circuit is provided to prevent an ignition circuit from being operated when a revolution per minute of the engine is higher than a critical one. However, such ignition systems cannot reduce the r.p.m. of the engine when an operator returns a throttle valve operating member in order to avert danger because it takes substantial time to reduce the r.p.m. of the engine. Furthermore, in a snowmobile having such ignition systems provided, even though the throttle valve operating member is returned, a throttle valve cannot be sometimes returned because a throttle wire freezes. This prevent the r.p.m. of the engine from being reduced.